pH Effects on the Selectivity of the Electrocatalytic CO₂ Reduction on Graphene-Embedded Fe–N–C Motifs: Bridging Concepts between Molecular Homogeneous and Solid-State Heterogeneous Catalysis

Abstract

In the past decades, the surplus of atmospheric CO2 concentration has drawn tremendous political and scientific attention for its negative impacts, such as the greenhouse effect and ocean carbonation. To mitigate such CO2 issues, a combination of various strategies is required. The electrochemical CO2 reduction reaction (CO2RR) is a promising alternative to convert CO2 into carbon-based chemicals and fuels, and electricity generated from the renewable sources (solar and wind) could be employed to sustain this transformation. At the current moment, the technological viability of this process is still contingent on finding affordable and efficient catalysts. In this thesis, a family of catalyst materials composed of abundant elements, in particular, non-precious metals, nitrogen, and carbon, typically referred to as precious group metal (PGM)-free "M-N-C" catalysts, were synthesized and mechanistically investigated-both experimentally and computationally-as catalyst candidates for the CO2RR. MNC catalysts feature hemoglobin-like single-site metallated porphyrin moieties with great impact on the catalytic reactivity and selectivity of the CO2RR. Among our studied M-N-C catalysts, the Ni-functionalized one exhibits great efficiency for CO yielding at high potentials and current densities. In particular, employment of Ni-N-C-based gas diffusion electrodes (GDE) combined with micro flow cells, allowed high CO evolution that could exceed 80% faradaic efficiency at 250 mA cm-2 current density, outperforming the industry commonly used Ag benchmark. By coupling our experimental observation and density functional theory (DFT) simulation, the reaction path from CO2 to CO over this sort single site catalyst could be deduced. Unlike the Ni-N-C catalyst, the Fe-N-C shows selective CO production only at low potentials. Further, due to relatively strong interaction with CO*, it opens the chance for hydrocarbons formation, yet showing little selectivity. To understand the mechanism behind this kind of selectivity, we carried out a series of studies, discussing catalytic tests, in-operando spectroscopic analysis, and computational modeling. Towards material research, operando-XAFS measurements identified an unusual Fe-N3, possibly a Fe I-N3 state, which appears to enable CH4 evolution. Further mechanistic studies included the electrocatalytic reduction of CO and CH2O as possible reactive intermediates for CH4 production. By combining the experimental and computational results, we suggest a reaction network for CO2 reduction into a variety of carbon-based products over the Fe-N-C catalyst. This contributes to the overall mechanistic understanding of CO2RR over the M-N-C catalysts and delivers perspectives to evolve and design novel catalysts to produce hydrocarbons of high value.

Optimizing FeNC Materials as Electrocatalysts for the CO2 Reduction Reaction: Heat‐Treatment Temperature, Structure and Performance Correlations

“…Most of the CO2RR studies are carried out in 0.1 M KHCO 3 electrolyte which has a lower buffer capacity resulting in a higher local pH in comparison with KH 2 PO 4 / K 2 HPO 4 buffer. [24] By contrast, the selectivity towards CH 4 is higher than that reported on other MNC catalysts (Table S1). [24] Our results, however, are comparable with those obtained in KH 2 PO 4 /K 2 HPO 4 buffer.…”

“…[13] The activity, however, was dramatically increased by incorporation of a metal to the NÀ C structure. [24] Combined DFT and experimental studies have found that the high binding energy of the FeN x centers towards CO and CO 2 is responsible for the early CO onset potential on FeNC catalyst. Furthermore, the metal center has been shown to have a crucial role on determining the catalytic performance.…”

Catalytic Mechanisms and Design Principles for Single‐Atom Catalysts in Highly Efficient CO2 Conversion

“…[32][33][34] We therefore define the applied potential to overcome the maximum energy barriers as the overpotentials (η) for CO 2 RR or HER. [32][33][34] We therefore define the applied potential to overcome the maximum energy barriers as the overpotentials (η) for CO 2 RR or HER.…”

The electronic structure underlying electrocatalysis of two‐dimensional materials

“…Combined with experimental measurements and DFT calculations, they found that at low overpotential regime (>−0.45 eV), potential‐determining step is the first proton‐coupled electron transfer reduction of CO 2 to adsorbed COOH; at the intermediate overpotential regime around −0.6 Vs RHE, CO turn‐over frequency (TOF) is linearly related to the CO binding energy: stronger CO binding corresponds to higher CO TOF, indicating COOH* → CO* being the rate‐determining step; at large overpotential regime <−0.7 versus RHE, Co‐ and Mn‐based catalysts start to strongly catalyze the hydrogen evolution reaction, while the Ni‐ and Cu‐based catalysts exhibit very weak binding of H* which makes the HER unfavorable and both high CO selectivity and reactivity can be obtained for Ni‐based catalysts. Meanwhile, Varela et al focused on the origin of the pH effects on the selectivity. They found that the CO formation rate is independent of the pH at fixed potential versus NHE, in consistency with a proton‐decoupled electron‐transfer pathway in which the reduction of CO 2 to CO occurs via a CO 2 − * intermediate, suggesting a decoupled elementary proton−electron transfer mechanism (Figure b,c).…”